Patent Document 1 discloses a battery module that has battery cells connected by bus bars. In Patent Document 1, battery cells are arranged side by side such that connection terminals of different polarities are adjacent to each other. The battery cells are connected in series by connecting adjacent connection terminals with a bus bar.
The battery cells expand as films form on the electrodes due to long-term use. Expansion of the battery cells increases the distance between adjacent battery cells, resulting in stress in the bus bars. The stress in the bus bars may cause the bus bars to come off the battery cells or increase the contact resistance between the battery cells and the bus bars. As a result, the connection state between the bus bars and the battery cells may not be maintained satisfactorily.